The movement of water and solute between blood, brain and cerebrospinal fluid (CSF) will be studied under conditions of altered CSF volume flow. During steady state perfusion or the cerebral ventricles of cats, CSF flow will be increased of decreased either by the intravenous infusion of venticular perfusion of anisotonic sucrose solutions. The effects of these induced osmotic gradients of brain water content will also be measured. In these experiments, the source of increased CSF bulk flow and sodium either from the extracellular space of the brain or from the blood in the chorid plexus will be sought. CSF formation by the choroid plexus, can be inhibited partially with acetazolamide or completely with a 30 mOsm perfusion solution, and the flow of fluid from the other source measured. The hypothesis that the sink action of CSF for water prevents and excessive accumulation during water intoxication can be tested. In addition, blood flow in the choroid plexus will be measured to determine whether increases in bulk flow can also be attributed to increases in blood flow through this organ. BIBLIOGRAPHIC REFERENCES: A. Wald, G. M. Hochwald, C. Mahan. The relationship between sodium influx and volum flow into the cerebral ventricles of cats. J. of Neurochem., 1975, 25, 151-154. G. M. Hochwald. A. wald, and C. Malhan. Sodium movement during extremes of cerebrospinal fluid formation. Amer. Acad. of Neurol., 1975, 25, 381.